Module 6 (chapter 25) - aromatic compounds Flashcards
background of benzene
- colourless, sweet smelling, flammable liquid
- found naturally in crude oil, a component of petrol and found in cigarette smoke
- classified as a carcinogen
- hexagonal rings of six carbon atoms with each carbon joined to one hydrogen atom
- classed as an arene
derivatives of benzene
- benzaldehyde has the flavour of almonds
- thymol is found in the aromatic herb, thyme
Kekule’s model
- 6 carbon joined by alternating single and double carbon bonds
how does the lack of reactivity disprove Kekule’s modern
- if benzene contained a C=C bond it should declourise bromine in an electrophilic addition reaction
- benzene does not decolourise bromine under normal conditions
- this led scientists to suggest that benzene cannot have any C=C bonds in its structure
how does the length of carbon-carbon bonds disprove Kekule’s theory
- X-ray diffraction used to measure bond lengths
- in benzene all bounds were found to be the same length (0.139nm)
- this was between the length of the single bond (0.153nm and a double bond of 0.134nm)
how does hydrogenation enthalpies disprove Kekule’s theory
- if benzene did have the Kekule’s structure, then it would be expected to have an enthalpy change of hydrogenation that is three time that of cyclohexane
- when cyclohexane is hydrogenated, one double bond reacts with hydrogen
- the enthalpy change of hydrogenation is -120KJmol-1 an d so expected enthalpy is 360Kjmol-1
- the actual enthalpy change is hydrogenation of benzene is only 208kjmol-1
- this shows that the actual structure of benzene is more stable than the theoretical kekule’s model of benzene
the delocalised model of Benzene
- benzene is planar
- each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and one to a hydrogen atom
- each carbon atom has one electron in a a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
- adjacent p-orbitals electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density
- this overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure
- the six electrons occupying this system of pi-bonds are said to be delocalised
compounds with one substituent group
- in aromatic compounds, the benzene ring is often considered to be the parent chain
- alkyl groups, halogens and nitro groups are all considered the prefixes to benzene
- when a benzene ring is attached to an alkyl chain with a functional group or an alkyl chain with seven or more carbon atoms, benzene is considered a substituent
- this means the prefix phenyl is used
exceptions to substituent rule
- benzoic acid
- phenylamine
- benzaldeyde
compounds with more than one substituent group
- this means the ring is now numbered
- substituent groups are listed in alphabetical order using the smallest numbers possible
benzene reactions
- most are electrophilic substitution
- hydrogen atom is replaced by another atom or group of atoms
nitration of benzene
- benzene reacts slowly with nitric acid to form nitrobenzene
- this reaction is catalysed by sulphuric acid and heated at 50 degrees
- water bath used to maintain a steady temperature
- one of the hydrogen atoms is replaced by a nitro group (NO2)
why must nitration happen at 50 degrees
-if it goes above 50 degrees, further substitution reactions may occur leading to the production of dinitrobenzene
uses of nitrobenzene
- preparation of dyes, pharmaceuticals and pesticides
- e.g. starting material to make paracetamol
electrophile in nitration of benzene
-nitric acid isn’t the electrophile
STEP 1:it is the nitronium ion NO2+ produced by the reaction of concentrated nitric acid with concentrated sulphuric acid
STEP 2: the electrophile accepts a pair of electrons from the benzene ring to forma a dative covalent bond
-the organic intermediate formed is unstable and breaks down to form the organic product nitrobenzene and a H+ion
STEP 3:H+ ion reacts with HSO4- ion from step 1 to regenerate the catalyse
halogenation of benzene
- the halogens do not react with benzene unless a catalyst called a halogen carrier is present
- common carriers are AlCl3, FeCl3, AlBr3, FeBr3
- can be generated in situ from the metal and halogen
bromination of benzene
-room temperature and in presence of a halogen carrier
-one of the hydrogen atoms is replaced by a bromine atom
STEP 1: benzene is too stable to react with a non-polar bromine molecule. the electrophile is the bromonium ion, Br+ which is generated when the electron carrier catalyst reacts with bromine
STEP 2: the. bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. the organic intermediate is unstable and breaks down to form the organic product bromobenzene and an H+ ion
STEP 3: the H+ ion formed in step 2 reacts with the FeBr4- ion to regenerate the FeBr3 catalyst
chlorination of benzene
-reacts in the same way as bromine but the halogen carrier used is AlCl3 of FeCl3…
alkylation of benzene
- the substitution of a hydrogen atom in the benzene ring by an alkyl group.
- reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst, generating the electrophile
- sometime called Friedal crafts alkylation
acylation reactions
- when benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed
- for example when ethanoyl chloride reacts with benzene it forms penylethanone (used in perfume)
does cyclohexane decolourise bromine
-yes bromine adds across the double bond
-the pi-bond in the alkene contains localised electrons above and below the plane of the two bonded carbon atoms
this produces an area go high electron density
-the localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom slightly negative
-the slightly positive bromine atom enables the bromine molecule the act like an electrophile
benzene doesn’t react like cyclohexane
- doesn’t react with bromine unless a halogen catalyse is present
- this is because benzene has a delocalised pi-electron spread above and below the plane of the carbon atoms in the ring structure
- the electron density around any two carbon atoms in the benzene ring is less than the C=C double bond in an alkene
phenols
class of aromatic compounds hydroxyl group directly bonded to the benzene ring -an OH group bonded to a carbon side chain doesn't count
phenol as a weak acid
- less soluble in water than alcohols due to the presence of the no-polar benzene ring
- large and complex benzene ring it is difficult to tesselate and form intermolecular bonds
- seen by comparing the acid dissociation constant -when dissolved in water it partially dissociates forming the phenoxide ions and a proton
- because of this ability it is classified as a weak acid.
